High numerical aperture objective lens system

ABSTRACT

An objective lens system having a high numerical aperture, a large working distance, and low optical aberrations over a wide spectral band of wavelengths is disclosed. The objective lens system includes a first lens group, a second lens group, and a third lens group. The first lens group includes first and second positive meniscus lenses that are positioned at a distance from each other along an optical axis of the objective lens system. The distance may be dependent on a focal length of the objective lens system. The second lens group includes first and second meniscus lenses and a bi-convex lens. The third lens group includes a bi-concave lens and a doublet lens.

This application incorporates by reference in their entireties U.S.patent application Ser. No. 14/819,335, and U.S. provisional application62/056,701.

FIELD

The present disclosure relates to various configurations of an objectivelens system that may be used in, for example, an inspection system of alithographic apparatus.

BACKGROUND

A lithographic apparatus is a machine that applies a desired patternonto a target portion of a substrate. Lithographic apparatus can beused, for example, in the manufacture of integrated circuits (ICs). Inthat circumstance, a patterning device, which is alternatively referredto as a mask or a reticle, may be used to generate a circuit patterncorresponding to an individual layer of the IC, and this pattern can beimaged onto a target portion (e.g., comprising part of, one or severaldies) on a substrate (e.g., a silicon wafer) that has a layer ofradiation-sensitive material (resist). In general, a single substratewill contain a network of adjacent target portions that are successivelyexposed. Known lithographic apparatus include so-called steppers, inwhich each target portion is irradiated by exposing an entire patternonto the target portion in one go, and so-called scanners, in which eachtarget portion is irradiated by scanning the pattern through the beam ina given direction (the “scanning”-direction) while synchronouslyscanning the substrate parallel or anti parallel to this direction. Itis also possible to transfer the pattern from the patterning device tothe substrate by imprinting the pattern onto the substrate.

In lithographic processes, the patterned substrate and/or the reticlemay be inspected for, e.g., process control and verification. There arevarious techniques for performing such inspection, including the use ofscanning electron microscopes, and various specialized inspectionsystems, which may be used to detect defects on the reticle and/ormeasure, for example, critical dimension (CD) of the patterns on thesubstrate, overlay error between successive layers formed on thesubstrate. One type of specialized inspection system is a scatterometerin which a radiation beam is directed onto a target of the pattern onthe surface of the substrate and one or more properties of the scatteredor reflected radiation beam, for example, intensity at a single angle ofreflection as a function of wavelength, intensity at one or morewavelengths as a function of reflected angle, or polarization as afunction of reflected angle are measured to obtain a spectrum from whicha property of interest of the target may be determined. Determination ofthe property of interest may be performed by various techniques, such asbut not limited to reconstruction of the target structure by iterativeapproaches (e.g., rigorous coupled wave analysis or finite elementmethods), library searches, and/or principal component analysis. Twomain types of scatterometer are known. Spectroscopic scatterometer thatdirects a broadband radiation beam onto the substrate and measure thespectrum (intensity as a function of wavelength) of the radiation beamscattered into a particular narrow angular range. Angularly resolvedscatterometer that use a monochromatic radiation beam and measure theintensity of the scattered radiation beam as a function of angle.

Objective lens systems are used in these scatterometers for directingand/or focusing the radiation beam onto the object of inspection (e.g.,the reticle, the target of the pattern on the surface of the substrate)and for collecting and/or imaging the scattered or reflected light fromthe object of inspection. The amount of information obtained from thecollected light and/or from the images of the object may depend on thenumerical aperture (NA) of the objective lens system and the wavelengthsof the radiation beam used in the scatterometers. The higher the NA ofthe objective lens system and the wider the spectral band of wavelengthsused in the scatterometers, the greater is the amount of informationthat can be obtained from the illuminated object of inspection. However,the highest NA and the maximum spectral bandwidth that can be used in aninspection system are limited by the design and configuration of one ormore lenses in the objective lens system.

There are three types of high NA objective lens systems currently usedfor scatterometry applications: refractive, reflective, andcatadioptric. Certain disadvantages are associated with the use of thesecurrent objective lens systems. One of the disadvantages of current highNA refractive objective lens system is that the working distance isrelatively small. For example, the working distance is generally lessthan 0.35 mm for high NA (e.g., 0.9-0.95). Another one of thedisadvantages is that the spectral band of wavelengths over which thecurrent high NA refractive objective lens system can operate withoutcompromising optical performance is limited to wavelengths ranging fromabout 450-700 nm. Use of current refractive objective lens systemsoutside this spectral band of wavelengths (e.g., below 450 nmwavelength, above 700 nm wavelength, between 410-450 nm wavelengths,between 700-900 nm wavelengths, at deep ultra violet (DUV) wavelengths,at infrared (IR) wavelengths) results in a loss of resolution due tochromatic aberrations (axial color aberrations). Loss of resolution canlead to reduced accuracy of the scatterometer measurements.

One of the disadvantages of current catadioptric and/or reflectiveobjective lens systems is that they have a large Petzval sum that is farfrom zero (i.e., they do not have a flat field curvature) and as aresult induce field curvature aberration. Pupil aberration is anotherone of the disadvantages of the current catadioptric and/or reflectiveobjective lens systems due to their large field curvature and pupilsize. Further, the current catadioptric and/or reflective objective lenssystems suffer from obscuration that reduces the amount of collectedlight, and hence, the amount of information that can be collected fromthe object of inspection.

SUMMARY

Accordingly, there is a need for an improved objective lens system thatcan be configured to have a high NA without the above mentioneddisadvantages.

According to an embodiment, an objective lens system includes a firstlens group comprising first and second positive meniscus lenses that maybe positioned at a distance from each other along an optical axis of theobjective lens system. The distance may be dependent on a focal lengthof the objective lens system. The objective lens system further includesa second lens group comprising a triplet lens. The triplet lens maycomprise a first meniscus lens having a first surface and a secondsurface, a second meniscus lens having a third surface and a fourthsurface, and a bi-convex lens having a fifth surface and a sixthsurface. The third surface may be in substantial contact with the secondsurface and the fifth surface may be in substantial contact with thefourth surface. The objective lens system may further include a thirdlens group comprising a bi-concave lens and a doublet lens.

In another embodiment, an inspection system may be configured to measurea property of a substrate. The inspection system includes a radiationsource that may be configured to produce a radiation beam, an opticalsystem that may be configured to focus the radiation beam on to asurface of the substrate, and a detector that may be configured todetect the radiation beam reflected from the surface of the substrate.The optical system may comprise a first lens group comprising first andsecond positive meniscus lenses that may be positioned at a distancefrom each other along an optical axis of the objective lens system. Thedistance may be dependent on a focal length of the objective lenssystem. The optical system may further comprise a second lens groupcomprising a third positive meniscus lens, a negative meniscus lenscemented to the third positive meniscus lens, and a bi-convex lenscemented to the negative meniscus lens. The optical system may alsocomprise a third lens group comprising a bi-concave lens and a doubletlens.

Yet in another embodiment, a lithographic apparatus includes anillumination optical system that may be configured to illuminate apattern of a patterning device, a projection system that may beconfigured to project an image of the pattern on to a target portion ofa substrate, and an inspection apparatus that may be configured tomeasure a property of the substrate includes an objective lens system.The objective lens system may comprise a first lens group that may beconfigured to correct field curvature aberrations and pupil aberrationsof the objective lens system. The first lens group may comprise abi-concave lens and a doublet lens.

In a further embodiment, an objective lens system includes first andsecond meniscus lenses positioned at a distance from each other along anoptical axis of the objective lens system. The distance may be dependenton a focal length of the objective lens system. The objective lenssystem may further include third and fourth meniscus lenses in contactwith each other, a bi-convex lens in contact with the fourth meniscuslens, a triplet lens, and an aperture stop between the triplet lens andthe bi-convex lens.

Further features and advantages of the invention, as well as thestructure and operation of various embodiments of the invention, aredescribed in detail below with reference to the accompanying drawings.It is noted that the invention is not limited to the specificembodiments described herein. Such embodiments are presented herein forillustrative purposes only. Additional embodiments will be apparent topersons skilled in the relevant art(s) based on the teachings containedherein.

BRIEF DESCRIPTION OF THE DRAWINGS/FIGURES

The accompanying drawings, which are incorporated herein and form partof the specification, illustrate the present invention and, togetherwith the description, further serve to explain the principles of theinvention and to enable a person skilled in the relevant art(s) to makeand use the invention.

FIG. 1A is a schematic illustration of a reflective lithographicapparatus according to an embodiment of the invention.

FIG. 1B is a schematic illustration of a transmissive lithographicapparatus according to an embodiment of the invention.

FIG. 2 is a more detailed schematic illustration of the reflectivelithographic apparatus, according to an embodiment of the invention.

FIG. 3 is a schematic illustration of a lithographic cell, according toan embodiment of the invention.

FIGS. 4 and 5 are schematic illustrations of scatterometers, accordingto various embodiments of the invention.

FIGS. 6 to 9 are schematic illustrations of a cross-sectional view ofrefractive objective lens systems, according to various embodiments ofthe invention.

The features and advantages of the present invention will become moreapparent from the detailed description set forth below when taken inconjunction with the drawings, in which like reference charactersidentify corresponding elements throughout. In the drawings, likereference numbers generally indicate identical, functionally similar,and/or structurally similar elements. The drawing in which an elementfirst appears is indicated by the leftmost digit(s) in the correspondingreference number. Unless otherwise indicated, the drawings providedthroughout the disclosure should not be interpreted as to-scaledrawings.

DETAILED DESCRIPTION

This specification discloses one or more embodiments that incorporatethe features of this invention. The disclosed embodiment(s) merelyexemplify the invention. The scope of the invention is not limited tothe disclosed embodiment(s). The invention is defined by the claimsappended hereto.

The embodiment(s) described, and references in the specification to “oneembodiment,” “an embodiment,” “an example embodiment,” etc., indicatethat the embodiment(s) described may include a particular feature,structure, or characteristic, but every embodiment may not necessarilyinclude the particular feature, structure, or characteristic. Moreover,such phrases are not necessarily referring to the same embodiment.Further, when a particular feature, structure, or characteristic isdescribed in connection with an embodiment, it is understood that it iswithin the knowledge of one skilled in the art to effect such feature,structure, or characteristic in connection with other embodimentswhether or not explicitly described.

Before describing such embodiments in more detail, however, it isinstructive to present an example environment in which embodiments ofthe present invention may be implemented.

Example Reflective and Transmissive Lithographic Systems

FIGS. 1A and 1B are schematic illustrations of a lithographic apparatus100 and lithographic apparatus 100′, respectively, in which embodimentsof the present invention may be implemented. Lithographic apparatus 100and lithographic apparatus 100′ each include the following: anillumination system (illuminator) IL configured to condition a radiationbeam B (for example, deep ultra violet or extreme ultra violetradiation); a support structure (for example, a mask table) MTconfigured to support a patterning device (for example, a mask, areticle, or a dynamic patterning device) MA and connected to a firstpositioner PM configured to accurately position the patterning deviceMA; and, a substrate table (for example, a wafer table) WT configured tohold a substrate (for example, a resist coated wafer) W and connected toa second positioner PW configured to accurately position the substrateW. Lithographic apparatus 100 and 100′ also have a projection system PSconfigured to project a pattern imparted to the radiation beam B bypatterning device MA onto a target portion (for example, comprising oneor more dies) C of the substrate W. In lithographic apparatus 100, thepatterning device MA and the projection system PS are reflective. Inlithographic apparatus 100′, the patterning device MA and the projectionsystem PS are transmissive.

The illumination system IL may include various types of opticalcomponents, such as refractive, reflective, catadioptric, magnetic,electromagnetic, electrostatic, or other types of optical components, orany combination thereof, for directing, shaping, or controlling theradiation beam B.

The support structure MT holds the patterning device MA in a manner thatdepends on the orientation of the patterning device MA with respect to areference frame, the design of at least one of the lithographicapparatus 100 and 100′, and other conditions, such as whether or not thepatterning device MA is held in a vacuum environment. The supportstructure MT may use mechanical, vacuum, electrostatic, or otherclamping techniques to hold the patterning device MA. The supportstructure MT can be a frame or a table, for example, which can be fixedor movable, as required. By using sensors, the support structure MT canensure that the patterning device MA is at a desired position, forexample, with respect to the projection system PS.

The term “patterning device” MA should be broadly interpreted asreferring to any device that can be used to impart a radiation beam Bwith a pattern in its cross-section, such as to create a pattern in thetarget portion C of the substrate W. The pattern imparted to theradiation beam B can correspond to a particular functional layer in adevice being created in the target portion C to form an integratedcircuit.

The patterning device MA may be transmissive (as in lithographicapparatus 100′ of FIG. 1B) or reflective (as in lithographic apparatus100 of FIG. 1A). Examples of patterning devices MA include reticles,masks, programmable mirror arrays, and programmable LCD panels. Masksare well known in lithography, and include mask types such as binary,alternating phase shift, and attenuated phase shift, as well as varioushybrid mask types. An example of a programmable mirror array employs amatrix arrangement of small mirrors, each of which can be individuallytilted so as to reflect an incoming radiation beam in differentdirections. The tilted mirrors impart a pattern in the radiation beam Bwhich is reflected by a matrix of small mirrors.

The term “projection system” PS can encompass any type of projectionsystem, including refractive, reflective, catadioptric, magnetic,electromagnetic and electrostatic optical systems, or any combinationthereof, as appropriate for the exposure radiation being used, or forother factors, such as the use of an immersion liquid on the substrate Wor the use of a vacuum. A vacuum environment can be used for EUV orelectron beam radiation since other gases can absorb too much radiationor electrons. A vacuum environment can therefore be provided to thewhole beam path with the aid of a vacuum wall and vacuum pumps.

Lithographic apparatus 100 and/or lithographic apparatus 100′ can be ofa type having two (dual stage) or more substrate tables WT (and/or twoor more mask tables). In such “multiple stage” machines, the additionalsubstrate tables WT can be used in parallel, or preparatory steps can becarried out on one or more tables while one or more other substratetables WT are being used for exposure. In some situations, theadditional table may not be a substrate table WT.

Referring to FIGS. 1A and 1B, the illuminator IL receives a radiationbeam from a radiation source SO. The source SO and the lithographicapparatus 100, 100′ can be separate physical entities, for example, whenthe source SO is an excimer laser. In such cases, the source SO is notconsidered to form part of the lithographic apparatus 100 or 100′, andthe radiation beam B passes from the source SO to the illuminator ILwith the aid of a beam delivery system BD (in FIG. 1B) including, forexample, suitable directing mirrors and/or a beam expander. In othercases, the source SO can be an integral part of the lithographicapparatus 100, 100′—for example when the source SO is a mercury lamp.The source SO and the illuminator IL, together with the beam deliverysystem BD, if required, can be referred to as a radiation system.

The illuminator IL can include an adjuster AD (in FIG. 1B) for adjustingthe angular intensity distribution of the radiation beam. Generally, atleast the outer and/or inner radial extent (commonly referred to as“σ-outer” and “σ-inner,” respectively) of the intensity distribution ina pupil plane of the illuminator can be adjusted. In addition, theilluminator IL can comprise various other components (in FIG. 1B), suchas an integrator IN and a condenser CO. The illuminator IL can be usedto condition the radiation beam B to have a desired uniformity andintensity distribution in its cross section.

Referring to FIG. 1A, the radiation beam B is incident on the patterningdevice (for example, mask) MA, which is held on the support structure(for example, mask table) MT, and is patterned by the patterning deviceMA. In lithographic apparatus 100, the radiation beam B is reflectedfrom the patterning device (for example, mask) MA. After being reflectedfrom the patterning device (for example, mask) MA, the radiation beam Bpasses through the projection system PS, which focuses the radiationbeam B onto a target portion C of the substrate W. With the aid of thesecond positioner PW and position sensor IF2 (for example, aninterferometric device, linear encoder, or capacitive sensor), thesubstrate table WT can be moved accurately (for example, so as toposition different target portions C in the path of the radiation beamB). Similarly, the first positioner PM and another position sensor IF 1can be used to accurately position the patterning device (for example,mask) MA with respect to the path of the radiation beam B. Patterningdevice (for example, mask) MA and substrate W can be aligned using maskalignment marks M1, M2 and substrate alignment marks P1, P2.

Referring to FIG. 1B, the radiation beam B is incident on the patterningdevice (for example, mask MA), which is held on the support structure(for example, mask table MT), and is patterned by the patterning device.Having traversed the mask MA, the radiation beam B passes through theprojection system PS, which focuses the beam onto a target portion C ofthe substrate W. The projection system has a pupil PPU conjugate to anillumination system pupil IPU. Portions of radiation emanate from theintensity distribution at the illumination system pupil IPU and traversea mask pattern without being affected by diffraction at a mask patternand create an image of the intensity distribution at the illuminationsystem pupil IPU.

With the aid of the second positioner PW and position sensor IF (forexample, an interferometric device, linear encoder, or capacitivesensor), the substrate table WT can be moved accurately (for example, soas to position different target portions C in the path of the radiationbeam B). Similarly, the first positioner PM and another position sensor(not shown in FIG. 1B) can be used to accurately position the mask MAwith respect to the path of the radiation beam B (for example, aftermechanical retrieval from a mask library or during a scan).

In general, movement of the mask table MT can be realized with the aidof a long-stroke module (coarse positioning) and a short-stroke module(fine positioning), which form part of the first positioner PM.Similarly, movement of the substrate table WT can be realized using along-stroke module and a short-stroke module, which form part of thesecond positioner PW. In the case of a stepper (as opposed to ascanner), the mask table MT can be connected to a short-stroke actuatoronly or can be fixed. Mask MA and substrate W can be aligned using maskalignment marks M1, M2, and substrate alignment marks P1, P2. Althoughthe substrate alignment marks (as illustrated) occupy dedicated targetportions, they can be located in spaces between target portions (knownas scribe-lane alignment marks). Similarly, in situations in which morethan one die is provided on the mask MA, the mask alignment marks can belocated between the dies.

Mask table MT and patterning device MA can be in a vacuum chamber, wherean in-vacuum robot IVR can be used to move patterning devices such as amask in and out of vacuum chamber. Alternatively, when mask table MT andpatterning device MA are outside of the vacuum chamber, an out-of-vacuumrobot can be used for various transportation operations, similar to thein-vacuum robot IVR. Both the in-vacuum and out-of-vacuum robots need tobe calibrated for a smooth transfer of any payload (e.g., mask) to afixed kinematic mount of a transfer station.

The lithographic apparatus 100 and 100′ can be used in at least one ofthe following modes:

1. In step mode, the support structure (for example, mask table) MT andthe substrate table WT are kept essentially stationary, while an entirepattern imparted to the radiation beam B is projected onto a targetportion C at one time (i.e., a single static exposure). The substratetable WT is then shifted in the X and/or Y direction so that a differenttarget portion C can be exposed.

2. In scan mode, the support structure (for example, mask table) MT andthe substrate table WT are scanned synchronously while a patternimparted to the radiation beam B is projected onto a target portion C(i.e., a single dynamic exposure). The velocity and direction of thesubstrate table WT relative to the support structure (for example, masktable) MT can be determined by the (de-)magnification and image reversalcharacteristics of the projection system PS.

3. In another mode, the support structure (for example, mask table) MTis kept substantially stationary holding a programmable patterningdevice, and the substrate table WT is moved or scanned while a patternimparted to the radiation beam B is projected onto a target portion C. Apulsed radiation source SO can be employed and the programmablepatterning device is updated as required after each movement of thesubstrate table WT or in between successive radiation pulses during ascan. This mode of operation can be readily applied to masklesslithography that utilizes a programmable patterning device, such as aprogrammable mirror array.

Combinations and/or variations on the described modes of use or entirelydifferent modes of use can also be employed.

In a further embodiment, lithographic apparatus 100 includes an extremeultraviolet (EUV) source, which is configured to generate a beam of EUVradiation for EUV lithography. In general, the EUV source is configuredin a radiation system, and a corresponding illumination system isconfigured to condition the EUV radiation beam of the EUV source.

FIG. 2 shows the lithographic apparatus 100 in more detail, includingthe source collector apparatus SO, the illumination system IL, and theprojection system PS. The source collector apparatus SO is constructedand arranged such that a vacuum environment can be maintained in anenclosing structure 220 of the source collector apparatus SO. An EUVradiation emitting plasma 210 may be formed by a discharge producedplasma source. EUV radiation may be produced by a gas or vapor, forexample Xe gas, Li vapor or Sn vapor in which the very hot plasma 210 iscreated to emit radiation in the EUV range of the electromagneticspectrum. The very hot plasma 210 is created by, for example, anelectrical discharge causing an at least partially ionized plasma.Partial pressures of, for example, 10 Pa of Xe, Li, Sn vapor or anyother suitable gas or vapor may be required for efficient generation ofthe radiation. In an embodiment, a plasma of excited tin (Sn) isprovided to produce EUV radiation.

The radiation emitted by the hot plasma 210 is passed from a sourcechamber 211 into a collector chamber 212 via an optional gas barrier orcontaminant trap 230 (in some cases also referred to as contaminantbarrier or foil trap) which is positioned in or behind an opening insource chamber 211. The contaminant trap 230 may include a channelstructure. Contamination trap 230 may also include a gas barrier or acombination of a gas barrier and a channel structure. The contaminanttrap or contaminant barrier 230 further indicated herein at leastincludes a channel structure, as known in the art.

The collector chamber 212 may include a radiation collector CO which maybe a so-called grazing incidence collector. Radiation collector CO hasan upstream radiation collector side 251 and a downstream radiationcollector side 252. Radiation that traverses collector CO can bereflected off a grating spectral filter 240 to be focused in a virtualsource point IF. The virtual source point IF is commonly referred to asthe intermediate focus, and the source collector apparatus is arrangedsuch that the intermediate focus IF is located at or near an opening 219in the enclosing structure 220. The virtual source point IF is an imageof the radiation emitting plasma 210. Grating spectral filter 240 isused in particular for suppressing infra-red (IR) radiation.

Subsequently the radiation traverses the illumination system IL, whichmay include a facetted field mirror device 222 and a facetted pupilmirror device 224 arranged to provide a desired angular distribution ofthe radiation beam 221, at the patterning device MA, as well as adesired uniformity of radiation intensity at the patterning device MA.Upon reflection of the beam of radiation 221 at the patterning deviceMA, held by the support structure MT, a patterned beam 226 is formed andthe patterned beam 226 is imaged by the projection system PS viareflective elements 228, 230 onto a substrate W held by the wafer stageor substrate table WT.

More elements than shown may generally be present in illumination opticsunit IL and projection system PS. The grating spectral filter 240 mayoptionally be present, depending upon the type of lithographicapparatus. Further, there may be more mirrors present than those shownin the FIGs., for example there may be 1-6 additional reflectiveelements present in the projection system PS than shown in FIG. 2.

Collector optic CO, as illustrated in FIG. 2, is depicted as a nestedcollector with grazing incidence reflectors 253, 254 and 255, just as anexample of a collector (or collector mirror). The grazing incidencereflectors 253, 254 and 255 are disposed axially symmetric around anoptical axis O and a collector optic CO of this type is preferably usedin combination with a discharge produced plasma source, often called aDPP source.

Example Lithographic Cell

FIG. 3 shows a lithographic cell 300, also sometimes referred to alithocell or cluster. Lithographic apparatus 100 or 100′ may form partof lithographic cell 300. Lithographic cell 300 may also includeapparatus to perform pre- and post-exposure processes on a substrate.Conventionally these include spin coaters SC to deposit resist layers,developers DE to develop exposed resist, chill plates CH and bake platesBK. A substrate handler, or robot, RO picks up substrates frominput/output ports I/O1, I/O2, moves them between the different processapparatus and delivers then to the loading bay LB of the lithographicapparatus. These devices, which are often collectively referred to asthe track, are under the control of a track control unit TCU which isitself controlled by the supervisory control system SCS, which alsocontrols the lithographic apparatus via lithography control unit LACU.Thus, the different apparatus can be operated to maximize throughput andprocessing efficiency.

Example Scatterometers

In order to ensure that the substrates that are exposed by alithographic apparatus, such as lithographic apparatus 100 and/or 100′are exposed correctly and consistently, it is desirable to inspectexposed substrates to measure properties such as overlay errors betweensubsequent layers, line thicknesses, critical dimensions (CD), etc. Iferrors are detected, adjustments may be made to exposures of subsequentsubstrates, especially if the inspection can be done soon and fastenough that other substrates of the same batch are still to be exposed.Also, already exposed substrates may be stripped and reworked—to improveyield—or discarded, thereby avoiding performing exposures on substratesthat are known to be faulty. In a case where only some target portionsof a substrate are faulty, further exposures can be performed only onthose target portions which are good.

An inspection apparatus may be used to determine the properties of thesubstrates, and in particular, how the properties of differentsubstrates or different layers of the same substrate vary from layer tolayer. The inspection apparatus may be integrated into a lithographicapparatus, such as lithographic apparatus 100 and/or 100′ or lithocell300 or may be a stand-alone device. To enable most rapid measurements,it is desirable that the inspection apparatus measure properties in theexposed resist layer immediately after the exposure. However, the latentimage in the resist has a very low contrast—there is only a very smalldifference in refractive index between the parts of the resist whichhave been exposed to radiation and those which have not—and not allinspection apparatus have sufficient sensitivity to make usefulmeasurements of the latent image. Therefore measurements may be takenafter the post-exposure bake step (PEB) which is customarily the firststep carried out on exposed substrates and increases the contrastbetween exposed and unexposed parts of the resist. At this stage, theimage in the resist may be referred to as semi-latent. It is alsopossible to make measurements of the developed resist image—at whichpoint either the exposed or unexposed parts of the resist have beenremoved—or after a pattern transfer step such as etching. The latterpossibility limits the possibilities for rework of faulty substrates butmay still provide useful information.

FIG. 4 depicts a scatterometer SM1 which may be used in the presentinvention. Scatterometer SM1 may be integrated into a lithographicapparatus, such as lithographic apparatus 100 and/or 100′ or lithocell300 or may be a stand-alone device. It comprises a broadband (whitelight) radiation projector 2 which projects radiation onto a substrateW. The reflected radiation is passed to a spectrometer detector 4, whichmeasures a spectrum 10 (intensity as a function of wavelength) of thespecular reflected radiation. From this data, the structure or profilegiving rise to the detected spectrum may be reconstructed by processingunit PU, e.g., by Rigorous Coupled Wave Analysis and non-linearregression or by comparison with a library of simulated spectra as shownat the bottom of FIG. 4. In general, for the reconstruction the generalform of the structure is known and some parameters are assumed fromknowledge of the process by which the structure was made, leaving only afew parameters of the structure to be determined from the scatterometrydata. Such a scatterometer may be configured as a normal-incidencescatterometer or an oblique-incidence scatterometer.

Another scatterometer SM2 that may be used with the present invention isshown in FIG. 5. Scatterometer SM2 may be integrated into a lithographicapparatus, such as lithographic apparatus 100 and/or 100′ or lithocell300 or may be a stand-alone device. Scatterometer SM2 may include anoptical system 1 having a radiation source 2, a lens system 12, a filter13 (e.g., interference filter), a reflecting device 14 (e.g., referencemirror), a lens system 15 (e.g., a microscopic objective lens system,also referred herein as objective lens system), a partially reflectedsurface 16 (e.g., a beam splitter), and a polarizer 17. ScatterometerSM2 may further include a detector 18 and a processing unit PU.

Objective lens system 15 may have a high numerical aperture (NA), e.g.,at least 0.9 or at least 0.95. Immersion scatterometers may even haveobjective lenses with numerical apertures over 1.

In one exemplary operation, the radiation emitted by radiation source 2is collimated using lens system 12 and transmitted through interferencefilter 13 and polarizer 17, is reflected by partially reflected surface16 and is focused onto substrate W via microscope objective lens system15. The reflected radiation then transmits through partially reflectingsurface 16 into a detector 18 in order to have the scatter spectrumdetected. The detector may be located in the back-projected pupil plane11, which is at the focal length of the objective lens system 15,however the pupil plane may instead be re-imaged with auxiliary optics(not shown) onto the detector. The pupil plane is the plane in which theradial position of radiation defines the angle of incidence and theangular position defines azimuth angle of the radiation. In one example,the detector is a two-dimensional detector so that a two-dimensionalangular scatter spectrum of a substrate target 30 can be measured. Thedetector 18 may be, for example, an array of CCD or CMOS sensors, andmay use an integration time of, for example, 40 milliseconds per frame.

A reference beam may be used, for example, to measure the intensity ofthe incident radiation. To do this, when the radiation beam is incidenton beam splitter 16 part of it is transmitted through the beam splitteras a reference beam towards reference mirror 14. The reference beam isthen projected onto a different part of the same detector 18 oralternatively on to a different detector (not shown).

Interference filter 13 may include a set of interference filters, whichmay be available to select a wavelength of interest in the range of,say, 405-790 nm or even lower, such as 200-300 nm. The interferencefilter may be tunable rather than comprising a set of different filters.A grating could be used instead of interference filters.

Detector 18 may measure the intensity of scattered light at a singlewavelength (or narrow wavelength range), the intensity separately atmultiple wavelengths or integrated over a wavelength range. Furthermore,detector 18 may separately measure the intensity of transverse magnetic-and transverse electric-polarized light and/or the phase differencebetween the transverse magnetic- and transverse electric-polarizedlight.

Using a broadband light source (i.e., one with a wide range of lightfrequencies or wavelengths—and therefore of colors) for a radiationsource 2 may give a large etendue, allowing the mixing of multiplewavelengths. The plurality of wavelengths in the broadband preferablyeach may have a bandwidth of Δλ and a spacing of at least 2Δλ (i.e.,twice the bandwidth). Several “sources” of radiation can be differentportions of an extended radiation source which have been split usingfiber bundles. In this way, angle resolved scatter spectra can bemeasured at multiple wavelengths in parallel. A 3-D spectrum (wavelengthand two different angles) can be measured, which contains moreinformation than a 2-D spectrum. This allows more information to bemeasured which increases metrology process robustness. This is describedin more detail in EP1,628,164A, which is incorporated by referenceherein in its entirety.

The target 30 on substrate W may be a 1-D grating, which is printed suchthat after development, the bars are formed of solid resist lines. Thetarget 30 may be a 2-D grating, which is printed such that afterdevelopment, the grating is formed of solid resist pillars or vias inthe resist. The bars, pillars or vias may alternatively be etched intothe substrate. This pattern is sensitive to chromatic aberrations in thelithographic projection apparatus, particularly the projection systemPL, and illumination symmetry and the presence of such aberrations willmanifest themselves in a variation in the printed grating. Accordingly,the scatterometry data of the printed gratings is used to reconstructthe gratings. The parameters of the 1-D grating, such as line widths andshapes, or parameters of the 2-D grating, such as pillar or via widthsor lengths or shapes, may be input to the reconstruction process,performed by processing unit PU, from knowledge of the printing stepand/or other scatterometry processes.

As described above, the target can be on the surface of the substrate.This target will often take the shape of a series of lines in a gratingor substantially rectangular structures in a 2-D array. The purpose ofrigorous optical diffraction theories in metrology is effectively thecalculation of a diffraction spectrum that is reflected from the target.In other words, target shape information is obtained for CD (criticaldimension) uniformity and overlay metrology. Overlay metrology is ameasuring system in which the overlay of two targets is measured inorder to determine whether two layers on a substrate are aligned or not.CD uniformity is simply a measurement of the uniformity of the gratingon the spectrum to determine how the exposure system of the lithographicapparatus is functioning. Specifically, CD, or critical dimension, isthe width of the object that is “written” on the substrate and is thelimit at which a lithographic apparatus is physically able to write on asubstrate.

Objective Lens System according to a First Embodiment

FIG. 6 illustrates a schematic of a cross-sectional view of a refractiveobjective lens system 600 that can be implemented as a part ofscatterometers SM1 and/or SM2 (shown in FIGS. 4 and 5), according to anembodiment. In an example of this embodiment, objective lens system 600may be used for directing and/or focusing a radiation beam emitted froman illumination system (not shown) onto an object of inspection (e.g.,patterning device MA, target 30 on substrate W, target portion C), andfor collecting and/or imaging the scattered or reflected light from theobject of inspection.

Objective lens system 600 may be configured to have a high NA (e.g., NAequal to about 0.95, NA greater than about 0.95, NA equal to about 1)without central obscuration, a large working distance (e.g., greaterthan about 0.35, greater than about 0.5) and low optical aberrations(e.g., low chromatic aberrations, low field curvature aberrations, lowpupil aberrations, low apochromatic aberrations) compared to currentobjective lens system. Additionally, objective lens system 600 may beconfigured to have a focal length ranging from about 3.5 vmm to about3.6 mm. Further, objective lens system 600 may be configured to operateover a wider spectral band of wavelengths (e.g., between 450-700 nmwavelengths, below 450 nm wavelength, above 700 nm wavelength, between410-450 nm wavelengths, between 700-900 nm wavelengths, at deep ultraviolet (DUV) wavelengths, at infrared (IR) wavelengths) compared tocurrent objective lens systems without compromising optical performance.

According to an example of this embodiment, objective lens system 600may comprise a front lens group 601, a middle lens group 602, and a backlens group 603. Front lens group 601, middle lens group 602, and backlens group 603 may be optically coupled to each other and may bearranged along optical axis 650 of objective lens system 600.

Front lens group 601 may be configured to decrease NA from an objectspace 656 to the entrance of middle lens group 602, according to anexample of this embodiment. For example, front lens group 601 maydecrease NA from about 0.95 in the object space to about 0.25-0.4 at theentrance of middle lens group 602. According to another example, frontlens group 601 may be configured to simultaneously correct or reducecoma aberrations and chromatic aberrations (axial color aberrations) ofobjective lens system 600. Such configurations of front lens group 601may be dependent on composition and arrangement of one or more lenses infront lens group 601.

In an example of this embodiment, front lens group 601 may comprise afirst positive meniscus lens 604 and a second positive meniscus lens 606that are optically coupled to each other (shown in FIG. 6). Firstpositive meniscus lens 604 may have a spherical concave surface 604 aand a spherical convex surface 604 b, and second positive meniscus lens606 may have a spherical concave surface 606 a and a spherical convexsurface 606 b. Convex surface 604 b may have a radius of curvature thatis smaller than a radius of curvature of convex surface 606 b. Convexsurface 604 b may be in substantial contact with concave surface 606 aat least at a point A along optical axis 650, according to an example.In another example, an air gap ranging from about 2% to about 8% of thefocal length of objective lens system 600 may be present between convexsurface 604 b and concave surface 606 a along optical axis 650.

First positive meniscus lens 604 may comprise, e.g., a heavy crownglass, a heavy flint glass, a lanthanum flint glass, a lanthanum denseflint glass, or a combination thereof having a refractive index greaterthan about 1.75, according to various examples of this embodiment. Inother examples of this embodiment, first positive meniscus lens 604 maycomprise an optical material having an Abbe number that ranges fromabout 45 to about 50, that is greater than 50, or that is greater than70.

One lens parameter is its Abbe number, which is a measure of thematerial's dispersion (variation of refractive index with wavelength) inrelation to the refractive index. High Abbe numbers indicate lowdispersion (low chromatic aberration), and vice versa. Second positivemeniscus lens 606 may comprise a heavy flint glass having an Abbe numberand a refractive index smaller than the Abbe number and refractive indexof first positive meniscus lens 604. For example, second positivemeniscus lens 606 may have an Abbe number less than about 30 and arefractive index that ranges from about 1.5 to about 1.6. The largedifference (e.g., greater than about 15) in the Abbe numbers of firstpositive meniscus lens 604 and second positive meniscus lens 606 mayallow the correction or reduction of coma and chromatic aberrations byfront lens group 601. For example, if first positive meniscus lens 604comprises an optical material having an Abbe number of about 45 andsecond positive meniscus lens 606 comprises an optical material havingan Abbe number of about 29, the difference in Abbe numbers is equal toabout 16.

In an embodiment, middle lens group 602 may be configured to correct orreduce apochromatic aberrations of objective lens system 600. Suchconfiguration of middle lens group 602 may be dependent on compositionand arrangement of one or more lenses in middle lens group 602. Middlelens group 602 may comprise a first doublet lens 608, a second doubletlens 610, a triplet lens 612, and a third doublet 614, as shown in FIG.6, according to an example of this embodiment.

First doublet lens 608 may comprise a bi-concave lens 616 and abi-convex lens 618 and may be positioned in a manner that firstbi-concave lens 616 is in substantial contact with second positivemeniscus lens 606 at least at a point B along optical axis 650, asillustrated in FIG. 6. Also, as illustrated in FIG. 6, bi-concave lens616 may have a thickness along optical axis 650 that is smaller than athickness of bi-convex lens 618 along optical axis 650. Bi-concave lens616 and bi-convex lens 618 may be coupled together and may havespherical surfaces. The coupling of bi-concave lens 616 and bi-convexlens 618 may be achieved by cementing these lenses to each other,according to an example. The lenses may be cemented by an adhesive(e.g., optically transparent epoxy) with mechanical strength to holdthese lenses together. In another example, bi-concave lens 616 andbi-convex lens 618 may be coupled by holding these lenses pressedagainst each other with external mounting fixtures because the opticaldesign may require an infinitesimal air gap between these lenses orbecause the difference in thermal expansion coefficients of these lensesdoes not allow cementing. The external mounting fixtures may hold theselenses together in a manner such that a partial or an entire surface ofbi-concave lens 616 is in substantial contact with a partial or anentire surface of bi-convex lens 618. Bi-concave lens 616 and bi-convexlens 618 may comprise materials such as, but not limited to, crown glassor flint glass. Both bi-concave lens 616 and bi-convex lens 618 maycomprise the same material or different material with respect to eachother.

Second doublet lens 610 may comprise a bi-concave lens 620 and abi-convex lens 622 and may be positioned in a manner that bi-concavelens 620 is in substantial contact with bi-convex surface 618 at leastat a point C along optical axis 650, as illustrated in FIG. 6. Also, asillustrated in FIG. 6, bi-concave lens 620 may have a thickness alongoptical axis 650 that is smaller than a thickness of bi-convex lens 622along optical axis 650. Bi-concave lens 620 and bi-convex lens 622 mayhave spherical surfaces and may be coupled together by cementing or byholding together, as described above with reference to bi-concave lens616 and bi-convex lens 618. Bi-concave lens 620 and bi-convex lens 622may comprise materials such as, but not limited to, crown glass materialor flint glass material. Both bi-concave lens 620 and bi-convex lens 622may comprise the same material or different material with respect toeach other.

Triplet lens 612 may comprise a positive meniscus lens 624, a negativemeniscus lens 626, and a bi-convex lens 628 and may be positioned in amanner that positive meniscus lens 624 is in substantial contact withbi-convex surface 622 at least at a point D along optical axis 650, asillustrated in FIG. 6. Positive meniscus lens 624, negative meniscuslens 626, and bi-convex lens 628 may have spherical surfaces and may becoupled together by cementing or by holding together, as described abovewith reference to bi-concave lens 616 and bi-convex lens 618.

In one example, bi-convex lens 628 may have a thickness along opticalaxis 650 that is greater than each thickness of positive meniscus lens624 and negative meniscus lens 626 along optical axis 650. In anotherexample, bi-convex lens 628 may have a thickness along optical axis 650that is greater than combined thickness of positive meniscus lens 624and negative meniscus lens 626 along optical axis 650. In a furtherexample, thickness of positive meniscus lens 624 and thickness ofnegative meniscus lens 626 along optical axis 650 is equal or differentwith respect to each other.

According to an example, positive meniscus lens 624 may comprise calciumfluoride (CaF₂), negative meniscus lens 626 may comprise a heavy crownglass, a heavy flint glass, a lanthanum flint glass, or a lanthanumdense flint glass, and bi-convex lens 628 may comprise a heavy flintglass having a refractive index that is greater than about 1.75. Thesecombinations of glass materials in triplet lens 612 may allow middlelens group 602 to correct or reduce apochromatic aberrations ofobjective lens system 600.

As further illustrated in FIG. 6, third doublet lens 614 may comprise abi-convex lens 630 and a positive meniscus lens 632 and may bepositioned in a manner that bi-convex lens 630 is in substantial contactwith bi-convex lens 628 at least at a point E along optical axis 650.Also, as illustrated in FIG. 6, bi-convex lens 630 may have a thicknessalong optical axis 650 that is greater than a thickness of positivemeniscus lens 632 along optical axis 650. Bi-convex lens 630 andpositive meniscus lens 632 may have spherical surfaces and may becoupled together by cementing or by holding together, as described abovewith reference to bi-concave lens 616 and bi-convex lens 618.

According to an embodiment, back lens group 603 may be configured tocorrect or reduce field curvature (also sometimes referred as Petzvalcurvature in the art) aberrations and pupil aberrations of objectivelens system 600. Such configuration of back lens group 603 may bedependent on composition and arrangement of one or more lenses in backlens group 603. Back lens group 603 may comprise a bi-convex lens 634and a doublet lens 636 that are placed adjacent to each other, but arenot in contact with each other, as shown in FIG. 6, according to anexample of this embodiment. Doublet lens 636 may comprise a bi-convexlens 638 and a bi-concave lens 640. Bi-convex lens 638 may have athickness along optical axis 650 that is greater than each thickness ofbi-concave lenses 634 and 640 along optical axis 650. Bi-convex lens 638and bi-concave lens 640 may have spherical surfaces and may be coupledtogether by cementing or by holding together, as described above withreference to bi-concave lens 616 and bi-convex lens 618.

Further, as illustrated in FIG. 6, objective lens system 600 maycomprise an aperture stop 652 and an entrance pupil 654 that may belocated along optical axis 650 and between middle lens group 602 andback group lens 603, according to an example of this embodiment.Location of entrance pupil 626 between middle lens group 602 and backlens group 618 may allow a diameter of aperture stop 624 to be adjusted.

Objective Lens System according to a Second Embodiment

FIG. 7 illustrates a schematic of a cross-sectional view of a refractiveobjective lens system 700 that can be implemented as a part ofscatterometers SM1 and/or SM2 (shown in FIGS. 4 and 5), according to anembodiment. Objective lens system 700 shares many similar features andconfigurations with objective lens system 600. Therefore, onlydifferences between objective lens systems 600 and 700 are to bediscussed below.

According to an example of this embodiment, objective lens system 700may comprise a front lens group 601, a middle lens group 602, and a backlens group 703. Front lens group 601, middle lens group 602, and backlens group 703 may be optically coupled to each other and may bearranged along optical axis 750 of objective lens system 700.

Back lens group 703 may comprise a bi-convex lens 707 interposed betweentwo bi-concave lenses 709 and 711, according to an example of thisembodiment. Bi-convex lens 638 and bi-concave lenses 709 and 711 mayhave spherical surfaces and may be coupled together to form a tripletlens by cementing or by holding together, as described above withreference to bi-concave lens 616 and bi-convex lens 618 (shown in FIG.6). Bi-convex lens 707 may have a thickness along optical axis 750 thatis greater than each thickness of bi-concave lenses 709 and 711 alongoptical axis 750. Such combination of lenses may allow back group lens703 to correct or reduce field curvature aberrations and pupilaberrations of objective lens system 700.

Objective Lens System according to a Third Embodiment

FIG. 8 illustrates a schematic of a cross-sectional view of a refractiveobjective lens system 800 that can be implemented as a part ofscatterometers SM1 and/or SM2 (shown in FIGS. 4 and 5), according to anembodiment. Objective lens system 800 shares many similar features andconfigurations with objective lens systems 600 and 700. Therefore, onlydifferences between objective lens systems 600, 700, and 800 are to bediscussed below.

According to an example of this embodiment, objective lens system 800may comprise a front lens group 801, a middle lens group 602, and a backlens group 703. Front lens group 801, middle lens group 602, and backlens group 703 may be optically coupled to each other and may bearranged along optical axis 850 of objective lens system 700.

Front lens group 801 is similar to front lens group 601 (shown in FIGS.6-7), except that front lens group 801 includes an aspherical meniscuslens 805 instead of the positive spherical meniscus lens 606 of frontlens group 601. Aspherical meniscus lens 805 may have an asphericalconcave surface 805 a and a spherical convex surface 805 b. Asphericalconcave surface 805 a may be in substantial contact with sphericalconvex surface 803 a at least at a point F along optical axis 850,according to an example. In another example, an air gap ranging fromabout 2% to about 8% of the focal length of objective lens system 800may be present between aspherical concave surface 805 a and sphericalconvex surface 803 a along optical axis 850. Presence of asphericalconcave surface 805 a in front lens group 801 may offer optimalaberration correction and as a result higher resolution compared tofront lens group 601 as aspherical surfaces inherently produce lessoptical aberrations (e.g., spherical aberrations) than sphericalsurfaces. In an example of this embodiment, aspherical meniscus lens 805may comprise a heavy flint glass having an Abbe number and a refractiveindex similar to the Abbe number and refractive index of second positivemeniscus lens 606 of front lens group 601.

Objective Lens System according to a Fourth Embodiment

FIG. 9 illustrates a schematic of a cross-sectional view of a refractiveobjective lens system 900 that can be implemented as a part ofscatterometers SM1 and/or SM2 (shown in FIGS. 4 and 5), according to anembodiment. Objective lens system 900 shares many similar features andconfigurations with objective lens systems 600, 700, and 800. Therefore,only differences between objective lens systems 600, 700, and 800 are tobe discussed below.

According to an example of this embodiment, objective lens system 900may comprise a front lens group 601, a middle lens group 902, and a backlens group 703. Front lens group 601, middle lens group 902, and backlens group 703 may be optically coupled to each other and may bearranged along optical axis 950 of objective lens system 900.

Middle lens group 902 is similar to middle lens group 602 (shown inFIGS. 6-8), except for the differences described herein. In an example,middle lens group 902 includes a concave lens 905 and a bi-convex lens907 forming a doublet lens 909 instead of the positive meniscus lens 632and the bi-convex lens 630 forming the doublet lens 614 of middle lensgroup 602. In another example, middle group lens 902 includes a tripletlens 911 having meniscus lenses 913 and 915 and a bi-convex lens 917.Radii of curvature of spherical surfaces of meniscus lenses 913 and 915are smaller than radii of curvature of the spherical surfaces ofmeniscus lenses 624 and 626 of middle lens group 602. These differencesin middle lens group 902 from middle lens group 602 may allow objectivelens system 900 to have a focal length smaller than the focal length ofobjective lens system 600, 700, and/or 800 for the same high NA as theobjective lens systems 600, 700, and 800. For example, objective lenssystem 900 may have a focal length of 2 mm for a NA of 0.95.

It should be noted that even though objective lens system 900 is shownto include a front lens group similar to front lens group 601 ofobjective lens 600 and a back lens group similar to back lens group 703of objective lens 700, objective lens system 900 may include front lensgroup that is similar to front lens group 801 of objective lens system800 and/or a back lens group that is similar to back lens group 603 ofobjective lens system 600, according to various examples of thisembodiment.

Although specific reference may be made in this text to the use anobjective lens system in inspection system, it should be understood thatthe objective lens system described herein may have other applicationsthat require a combination of high NA, large field of view (FOV), and/orwide spectral band.

Although specific reference may be made in this text to the use oflithographic apparatus in the manufacture of ICs, it should beunderstood that the lithographic apparatus described herein may haveother applications, such as the manufacture of integrated opticalsystems, guidance and detection patterns for magnetic domain memories,flat-panel displays, liquid-crystal displays (LCDs), thin-film magneticheads, etc. The skilled artisan will appreciate that, in the context ofsuch alternative applications, any use of the terms “wafer” or “die”herein may be considered as synonymous with the more general terms“substrate” or “target portion”, respectively. The substrate referred toherein may be processed, before or after exposure, in for example atrack (a tool that typically applies a layer of resist to a substrateand develops the exposed resist), a metrology tool and/or an inspectiontool. Where applicable, the disclosure herein may be applied to such andother substrate processing tools. Further, the substrate may beprocessed more than once, for example in order to create a multi-layerIC, so that the term substrate used herein may also refer to a substratethat already contains multiple processed layers.

Although specific reference may have been made above to the use ofembodiments of the invention in the context of optical lithography, itwill be appreciated that the invention may be used in otherapplications, for example imprint lithography, and where the contextallows, is not limited to optical lithography. In imprint lithography atopography in a patterning device defines the pattern created on asubstrate. The topography of the patterning device may be pressed into alayer of resist supplied to the substrate whereupon the resist is curedby applying electromagnetic radiation, heat, pressure or a combinationthereof. The patterning device is moved out of the resist leaving apattern in it after the resist is cured.

It is to be understood that the phraseology or terminology herein is forthe purpose of description and not of limitation, such that theterminology or phraseology of the present specification is to beinterpreted by those skilled in relevant art(s) in light of theteachings herein.

In the embodiments described herein, the terms “lens” and “lenselement,” where the context allows, can refer to any one or combinationof various types of optical components, including refractive,reflective, magnetic, electromagnetic, and electrostatic opticalcomponents.

Further, the terms “radiation” and “beam” used herein encompass alltypes of electromagnetic radiation, including ultraviolet (UV) radiation(for example, having a wavelength λ of 365, 248, 193, 157 or 126 nm),extreme ultraviolet (EUV or soft X-ray) radiation (for example, having awavelength in the range of 5-20 nm such as, for example, 13.5 nm), orhard X-ray working at less than 5 nm, as well as particle beams, such asion beams or electron beams. Generally, radiation having wavelengthsbetween about 780-3000 nm (or larger) is considered IR radiation. UVrefers to radiation with wavelengths of approximately 100-400 nm. Withinlithography, the term “UV” also applies to the wavelengths that can beproduced by a mercury discharge lamp: G-line 436 nm; H-line 405 nm;and/or, I-line 365 nm. Vacuum UV, or VUV (i.e., UV absorbed by gas),refers to radiation having a wavelength of approximately 100-200 nm.Deep UV (DUV) generally refers to radiation having wavelengths rangingfrom 126 nm to 428 nm, and in an embodiment, an excimer laser cangenerate DUV radiation used within a lithographic apparatus. It shouldbe appreciated that radiation having a wavelength in the range of, forexample, 5-20 nm relates to radiation with a certain wavelength band, ofwhich at least part is in the range of 5-20 nm.

The term “substrate” as used herein describes a material onto whichsubsequent material layers are added. In embodiments, the substrateitself may be patterned and materials added on top of it may also bepatterned, or may remain without patterning.

The term “in substantial contact” as used herein generally describeselements or structures that are in physical contact with each other withonly a slight separation from each other which typically results frommisalignment tolerances. It should be understood that relative spatialdescriptions between one or more particular features, structures, orcharacteristics (e.g., “vertically aligned,” “substantial contact,”etc.) used herein are for purposes of illustration only, and thatpractical implementations of the structures described herein may includemisalignment tolerances without departing from the spirit and scope ofthe present disclosure.

The term “optically coupled” as used herein generally refers to onecoupled element being configured to impart light to another coupledelement directly or indirectly.

The term “optical material” as used herein generally refers to amaterial that allows light or optical energy to propagate therein ortherethrough.

While specific embodiments of the invention have been described above,it will be appreciated that the invention may be practiced otherwisethan as described. The description is not intended to limit theinvention.

It is to be appreciated that the Detailed Description section, and notthe Summary and Abstract sections, is intended to be used to interpretthe claims. The Summary and Abstract sections may set forth one or morebut not all exemplary embodiments of the present invention ascontemplated by the inventor(s), and thus, are not intended to limit thepresent invention and the appended claims in any way.

The present invention has been described above with the aid offunctional building blocks illustrating the implementation of specifiedfunctions and relationships thereof. The boundaries of these functionalbuilding blocks have been arbitrarily defined herein for the convenienceof the description. Alternate boundaries can be defined so long as thespecified functions and relationships thereof are appropriatelyperformed.

The foregoing description of the specific embodiments will so fullyreveal the general nature of the invention that others can, by applyingknowledge within the skill of the art, readily modify and/or adapt forvarious applications such specific embodiments, without undueexperimentation, without departing from the general concept of thepresent invention. Therefore, such adaptations and modifications areintended to be within the meaning and range of equivalents of thedisclosed embodiments, based on the teaching and guidance presentedherein.

The breadth and scope of the present invention should not be limited byany of the above-described exemplary embodiments, but should be definedonly in accordance with the following claims and their equivalents.

The invention claimed is:
 1. An inspection system configured to measurea property of a substrate, the inspection system comprising: a radiationsource configured to produce a radiation beam; an optical systemconfigured to focus the radiation beam on to a surface of the substrate,the optical system comprising: a first lens group comprising: a firstpositive meniscus lens, and a second positive meniscus lens positionedat a distance from the first positive meniscus lens along an opticalaxis of the optical system, the distance being dependent on a focallength of the optical system, a second lens group comprising: a thirdpositive meniscus lens, a negative meniscus lens cemented to the thirdpositive meniscus lens, and a bi-convex lens cemented to the negativemeniscus lens, and a third lens group comprising: a bi-concave lens, anda doublet lens; and a detector configured to detect the radiation beamreflected from the surface of the substrate.
 2. The inspection system ofclaim 1, wherein the second positive meniscus lens comprises anaspherical concave surface.
 3. The inspection system of claim 1,wherein: the third positive meniscus lens comprises calcium fluoride;the negative meniscus lens comprises a heavy crown glass, a heavy flintglass, a lanthanum flint glass, or a lanthanum dense flint glass; andthe bi-convex lens comprises a heavy flint glass having a refractiveindex that is greater than about 1.75.
 4. The inspection system of claim1, wherein the bi-concave lens and the doublet are adjacent to andseparated from each other.
 5. The inspection system of claim 1, whereinthe bi-concave lens and the doublet are cemented to each other.
 6. Theinspection system of claim 1, wherein the third lens group is configuredto correct field curvature aberrations and pupil aberrations of theoptical system.